A magnetic recording medium is widely used for recording electromagnetic signals, and various shapes such as a tape, a disk or a card are used. A magnetic recording medium tape is used for audio recording, video recording and for computers (hereinafter, a magnetic recording medium tape is simply referred to as "magnetic tape").
Among magnetic tapes, tapes for audio and video recording are often enclosed in a cassette for use. Recently, a long recording time has been required for a magnetic tape enclosed in a cassette. Accordingly, there is a tendency for a magnetic tape to be made thinner and thinner, so that a tape which is as long as possible can be enclosed in a predetermined standard cassette.
In this connection, a magnetic tape provided with a backing layer has been developed and widely used to provide such a thin magnetic tape with strength and further to improve running efficiency by improving contact between the back surface of a magnetic tape and the running system of reproducing apparatus.
A backing layer generally comprises non-magnetic particles dispersed in a binder. The surface of the backing layer should not be uneven, to improve the running efficiency of a magnetic tape. That is, when the surface of the backing layer is extremely smooth, the area contacting the running system increases and the friction coefficient also increases, thereby decreasing the running efficiency of the magnetic tape. On the other hand, when unevenness is extremely great on the surface of the backing layer, upon preparing a magnetic tape, particularly in the step of heat treatment of a wound magnetic tape, unevenness on the surface of the backing layer prints through the magnetic layer contacting the surface of the backing layer (that is, notable unevenness is formed on the surface of the magnetic layer), thereby decreasing the electromagnetic characteristic of the magnetic tape. Accordingly, it is necessary not only to adjust the particle size distribution of non-magnetic particles but also to improve dispersibility of non-magnetic particles in a binder upon preparing a backing layer.
The effects of a binder upon surface properties of a backing layer have been studied, and it is disclosed, for example, in JP-A-56-98719 and U.S. Pat. No. 4,367,261 that a preferred binder for a backing layer is a combination of cellulose type resin, thermoplastic polyurethane elastomer and polyisocyanate. (The term "JP-A" as used herein used means an unexamined published Japanese patent application).
Such a binder is preferred with respect to the efficiency of a backing layer. However, there is a handling problem. Since nitrocellulose, which is the most typical cellulose resin, becomes explosive depending upon the handling conditions, particular attention must be paid in preparing a backing layer.
A polyurethane resin and a vinyl chloride type resin are used in combination as a binder to avoid using nitrocellulose. However, in the above binder system, dispersibility of non-magnetic particles is deteriorated, and thus unevenness on the surface of a backing layer tends to be extreme, resulting in deteriorated electromagnetic characteristics of the magnetic tape. There is also a problem that the thus-formed backing layer is fragile, so that cracks tend to take place and the edge of the tape tends to break.
There is also another problem that the surface electric resistance on a tape is high, because various materials used for preparing the tape are electrically insulating. That is, due to friction and the like, a magnetic tape is charged, and as a result, dust and contaminants easily attach to the surface of the tape, thereby causing drop outs; static friction coefficient increases, thereby disturbing the wound shape of a magnetic tape; and running properties deteriorate.
To avoid the above defects, it is proposed to reduce the surface electric resistance of a magnetic tape by adding electroconductive substances such as carbon black to a magnetic layer, and to add anion, cation, nonionic and amphoteric surface active agents. But, addition of excess amounts of electroconductive substances results in reducing magnetic flux density as well as sensitivity of a magnetic recording medium, and addition of excess amounts of surface active agents results in deteriorated running properties and increased viscosity of a tape.